Automatic speech recognition has only recently emerged from the research laboratory as a viable technology. Among the automatic speech recognition systems that have been developed are large-vocabulary document dictation systems and automated telephone directory systems. Large vocabulary dictation systems typically require the use of head-mounted close-talking microphones, a relatively quiet operating environment and a considerable amount of speaker adaptation. Telephone-based systems, on the other hand, are operable over a wide range of telephone channel conditions with relatively little or no speaker adaptation; however, users typically are limited with respect to the kinds of speech input that is recognizable. For example, such systems typically require discontinuous speech input or limit the grammar or vocabulary of the recognizable speech.
The performance of all automatic speech recognition systems degrades when acoustic interference is present in the input speech signal. Such interference may include one or more of the following: extraneous sounds (additive noise) received from the speaker's environment or the communication channel, spectral shaping or nonlinear distortion imposed by the microphone or communication channel, and reverberation from the room in which the speaker is talking.